1. Field of the Invention
The present invention relates to mixers and more particularly to low power mixers with improved linearity (IP3) performance.
2. Description of the Prior Art
Mixers are used in many commercial applications including commercial wireless applications, such as cellular telephones, cordless phones, global positioning systems and PCMCIA computer interface applications. Such applications require relatively high linearity (IP3) performance, a low noise figure and good isolation performance for frequencies less than 6 GHz. Such mixers also need to operate at relatively low dc voltages for example, less than 3.6 volts dc. Mixers used in portable dc electronics are required to have low dc power consumption to maximize battery life and also need to be relatively small in size. Many known mixers fail to accomplish all of these performance objectives.
Known Gilbert cell mixers can provide positive conversion gain, and good isolation and are relatively small area and are adapted to be monolithically integrated. Unfortunately, such known Gilbert cell mixers require relatively large amounts of dc current and power to achieve a fraction of the linearity performance of known Schottky diode mixer designs. In addition, such Gilbert cell mixers typically require about 2.5-3 V.sub.BE supply voltage in order to provide a reasonable linearity performance.
Conventional Schottky diode active mixer designs provide for relatively high linearity, good isolation and conversion loss performance while dissipating little or no dc power. However, conventional Schottky mixers which employ passive baluns consume relatively large areas, especially at frequencies &lt;2 GHz and are limited to about an octave of bandwidth.
Schottky diode mixers with active baluns are also known, for example as disclosed copending U.S. Pat. application, Ser. No. 08/500725, filed on Jul. 11, 1995 and "A Novel HBT Active Transformer Balanced Schottky Diode Mixer", by K. Kobayashi, 1996 IEEE MMT-S Dig., San Diego, Calif., hereby incorporated by reference. Such mixers are configured with a active balun topology to form a double balanced mixer which provides for relatively lower power supply voltage operation and higher linearity performance than known Gilbert cell mixers while maintaining a positive conversion gain, a relatively low LO drive requirement and double balanced multi-decade frequency performance. Such mixers also provide good amplitude and antiphase balance of &lt;.+-.1dB and &lt;.+-.6.degree. up to 5 GHz. Moreover, such mixers are also adapted to be monolithically integrated as a monolithic microwave integrated circuit (MMIC).
Referring to FIG. 3, the active balun mixer disclosed in the above mentioned patent application includes a Schottky diode ring quad and two active baluns. Each balun includes a single ended 50 ohm RF/LO input which includes a resistor R.sub.1, a transistor Q.sub.5 and current source I.sub.4. A mirror leg which includes a resistor R.sub.2. transistor Q.sub.6 and current source I.sub.5 is used to create a differential signal which drives a differential amplifier formed from the transistors Q.sub.1, Q.sub.2, a pair of low resistors R.sub.L1, R.sub.L2, and an emitter degeneration resistor R.sub.e, and current sources I.sub.1 and I.sub.2. The outputs of the collectors Q1 and Q2 provide the balanced antiphase complementary outputs. The emitter degeneration resistor R.sub.e is adapted to be adjusted to increase the input power handling range of the differential output amplifier as well as the conversion gain bandwidth performance. The values of the load resistors R.sub.L1 and RL.sub.2 are selected such that the product R.sub.L *I&gt;the turn on voltage of the Schottky diodes. In this manner, the balun can be driven by a large LO source which has enough voltage to switch on the Schottky diodes under large signal operation. Because the active LO and RF baluns are directly coupled to the Schottky diode ring-quad, the voltage product R.sub.L *I must be the same for both the RF and LO active baluns so that the diodes of the Schottky diode ring-quad are biased with 0 voltage bias in the absence of the RF and LO signals. This design constraint must be maintained in order to reserve the dc balance of the Schottky diodes as well as the active baluns. The IF center-tap is formed of a pair of transistors Q.sub.3 and Q.sub.4, configured as followers, and a current source I.sub.3. The transistors Q.sub.3, Q.sub. 4 combined in-phase IF signals at their emitters. The active balun is self-biased through a single supply voltage, V.sub.cc.
One inherent limitation in such a topology is that it does not efficiently utilize the full tail current of the differential amplifier which drives the Schottky diode ring-quad which limits performance as discussed below. More particularly, when half of the Schottky diode ring is switched on by either of the LO or RF active balun drivers, the maximum current through the Schottky diodes is limited to half of the tail current of the differential amplifier switch driver. In fact, in practice the actual Schottky diode current is known to be less than half of this tail current. It is known that the higher the LO drive and thus the higher the switch current through the Schottky diodes of the ring-quad, the better the linearity performance, conversion gain and noise figure performance of a mixer. Thus, the inefficient use of the tail current to drive the Schottky diode ring-quad will lead to a less than optimum mixer conversion gain, noise figure and linearity performance for the amount of dc current consumed by the balanced drivers.